Process for depositing layers of zirconium oxide using soluble powders
The invention relates to a process for depositing layers of zirconium oxide (ZrO2) on arbitrary substrates. In particular, the invention relates to a sol-gel process in which a soluble powder that can be redissolved in many solvents and solvent mixtures is firstly produced. The coating solution that is produced from the powder can be used in the production of, inter alia, layers of zirconium oxide on arbitrary substrates.
Technical interest in ZrO2 layers is principally aimed at the use thereof as anti-corrosive layers for steel at high temperatures or in relation to aggressive media. This is described in, for example, R. Di Maggio, P. Scardi and A. Tomasi, Material Engineering, 5,1 13-23 (1994); M. Atik and M. A. Aegerter, Mat. Res. Soc. Symp. Proc. Vol. 271 (1992) and M. Anast, J. M. Bell, T. J. Bell and B. Ben-Nissan, J. Mat. Sci. Let. 11 (1992).
In comparison with physical or physicochemical methods of deposition, such as sputtering, vaporisation, chemical vapour deposition and metal-organic decomposition, wet chemical production via the sol-gel process offers a wealth of advantages: no costly items of coating equipment incorporating vacuum technology are required, large areas can be coated easily, and the inside of cylindrical substrates is also readily accessible to coating solutions. In comparison with classical oxide-ceramic processes the necessary process temperatures are low, permitting the coating of metals or glass substrates. Such processes are described in Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing (Academic Press, Boston, 1990), for example.
Coating solutions that are employed in the sol-gel process are produced, as a rule, from metal alcoholates and transition-metal alcoholates. Since these compounds are not resistant to hydrolysis, their storability under normal conditions is limited. Chemical modification of the alkoxides by means of complexing agents also does not always result in a stabilisation of the solutions that is satisfactory for industrial applications.
One object underlying the invention is therefore to make available a process for depositing layers of zirconium oxide that does not exhibit the aforementioned disadvantages. A further object underlying the invention is to make available a process for the deposition of ZrO2 that enables deposition with a layer thickness of over 150 nm within one coating step. At the same time, the process is to permit the extensive coating of substrates of differing composition (various glasses, ceramic substrates, steel) and of variable geometry (flat specimens, tubes).
The invention provides a process for producing layers of zirconium oxide that comprises the following stages:
(i) production of a soluble, zirconium-containing powder by:
(a) reaction of a zirconium alcoholate of the general formula Zr(OR)4, in which the residues R are the same or different and represent straight-chain, branched or cyclic alkyl or alkenyl residues with 1 to 10 carbon atoms, which optionally exhibit one or more carbonyl and/or ester and/or carboxyl functions, with one or more polar compounds having complexing, chelating properties,
(b) heating the solution,
(c) mixing the solution with water, optionally in the presence of a catalyst,
(d) concentrating the solution until a powder is obtained,
(ii) dissolving the powder forming a sol,
(iii) coating a substrate with the sol, and
(iv) annealing the coated substrate.
According to a preferred embodiment of this process, zirconium alcoholates of the general formula Zr(OR)4 are employed in which R stands for a straight-chain or branched alkyl residue with 2 to 6 carbon atoms. Furthermore it is preferred if one or more OR residues of the aforementioned formula are derived from oxo esters, xcex2-diketones, carboxylic acids, ketocarboxylic acids or keto alcohols. It is particularly preferred if the OR residue is derived from acetylacetone. Examples of suitable zirconium alcoholates are Zr(OEt)4, Zr(Oi-Pr)4, Zr(On-Pr)4 and Zr(AcAc)2(Oi-Pr)2.
Instead of synthesising a coating solution directly, in accordance with the invention a soluble zirconium-containing powder is firstly produced. In this connection a zirconium alcoholate is caused to react with a polar, complexing and chelating compound. Such compounds are, for example, diketones, xcex2-keto esters, acetylacetone, glycol ethers, diols, polyhydric alcohols, amino alcohols, glycerol, hydroxydiols, aminothiols, dithiols, diamines or mixtures of the same.
Particularly preferred is the use of diketones, in particular 1,3-diketones such as acetylacetone.
With the process according to the invention the polar complexing and chelating compound is employed in a quantity from 0.5 to 20 mol, preferably 0.5 to 3 mol, per mol zirconium alcoholate.
After reaction of the zirconium alcoholate with the polar complexing and chelating compound, the solution that is obtained is heated to a temperature in the range from room temperature to the boiling-point of the solvent, preferably to 80 to 100xc2x0 C., over a time-interval of up to 24 hours, preferably over a time-interval from 0.5 to 2 hours.
Subsequently the solution is heated with a quantity from 0.5 to 20, preferably 1 to 3, mol H2O per mol zirconium alcoholate, optionally in the presence of a catalyst (H3O+, OH31 ) or with dilute inorganic or organic acids or lyes such as HNO3, HCl, NaOH or NH3, or with dilute solutions of metallic salts such as NaBF4, and the mixture is concentrated, preferably under reduced pressure. With this process a pulverulent solid is obtained which has a zirconium content from 30 to 55% by weight.
The powder according to the invention is capable of being stored in contact with air for an unlimited period.
The powder according to the invention can be redissolved in numerous solvents or solvent mixtures and can consequently be used for producing a coating solution for the deposition of layers of zirconium oxide, said coating solution also being a subject of the invention. Suitable solvents are alcohols, diols, diol ethers, amines, water and mixtures thereof. Through the choice of the solvents or solvent mixtures it is possible for important properties of the coating solution, such as solids content and viscosity, to be adjusted selectively and to be matched to the coating process and the substrate.
Suitable alcohols are aliphatic alcohols, in particular methanol, ethanol, isopropanol and butanol.
Suitable diol ethers are, e.g., methoxyethanol, butoxyethanol or isopropoxyethanol.
Suitable diols are those of the general formula HO(CH2)nOH in which n stands for an integer from 2 to 8, such as, e.g., ethanediol, propanediol, butanediol etc. Polyhydric alcohols or polyols such as, e.g., glycerin can also be employed in the process according to the invention.
Suitable amines are monoalkylamines, dialkylamines or trialkylamines, the alkyl groups of which may optionally be substituted by one or more OH groups. Preferred amines have the general formula NRxRxe2x80x23-x, in which R and Rxe2x80x2 may be the same or different and may stand for methyl, ethyl, hydroxyethyl and hydrogen and x signifies an integer from 0 to 3.
The-coating solutions according to the invention preferably contain water.
Quite particularly preferred are coating solutions that contain, by way of solvent, a mixture of propanediol, triethanolamine and water, the best results being achieved with coating solutions that contain the aforementioned solvents in a weight ratio of 60:10:30 with a solids content of 15% zirconium oxide.
Also preferred are coating solutions that contain, by way of solvent, a mixture of 1.5-pentanediol and ethanol, the best results being achieved with coating solutions that contain the aforementioned solvents in a weight ratio from 5/95 to 45/55 with a solids content of the sol amounting to 5-10% by weight with respect to ZrO2.
With a view to producing the coating solution according to the invention, the solvent or solvent mixture is mixed with the zirconium-containing powder. In this connection the solvent is preferably submitted and the powder is added in portions.
Subsequently the mixture is heated to a temperature in the range from room temperature to the boiling-point of the solvent mixture, preferably in the range from 80 to 100xc2x0 C., over a time-interval from 10 to 60 minutes, resulting in a colloidal solution (sol).
By addition of silicon alcoholates to the ZrO2 sols that have been produced in this way it is possible for the refractive index of the thin layers to be freely chosen in the range between 1.5 and 2.1. The preferred ratio of zirconium alcoholate to silicon alcoholate amounts to 9:91. Suitable silicon alcoholates which can be admixed are silicon alcoholates of the general formula Si(OR)4, in which the residues R are the same or different and represent straight-chain, branched-chain or cyclic alkyl or alkenyl residues with 1 to 10 carbon atoms, which optionally exhibit one or more carbonyl and/or ester and/or carboxyl functions.
Preferred silicon alcoholates are Si (OEt)4 and Si (OMe)4.
Depending on the mixing ratio of zirconium-containing powder to solvent or solvent mixture, the zirconium-oxide content of the sol amounts to 0.1 to 30%, preferably to about 15% (percent by mass).
A large number of substrate types can be wetted very well by the sols according to the invention. By single coating and subsequent annealing at a temperature of 550xc2x0 C. it is possible for ZrO2 layers with a thickness of up to 0.8 xcexcm to be deposited, depending on the coating process. Suitable coating processes are known as such to those skilled in the art and are described in Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, Academic Press (1990), for example.
Surprisingly, no step for drying the wet film is necessary when use is made of the coating solution according to the invention prior to thermal age-hardening. In the case of layer thicknesses above 0.4 xcexcm the layers exhibit a reticular fissured structure. Surprisingly, these layers exhibit good adhesion even on borosilicate glass.
If auxiliary substances such as polyethylene glycol are added to the coating solutions, it is possible for fine structures to be impressed into the layers (cf. N. Toghe et al., J. Non-Cryst. Solids, 100 (1988), 501).
Moreover, it is possible to introduce doping into the layers of zirconium oxide by introducing appropriate components into the solvent or solvent mixture in the course of production of the coating solution according to the invention. Such components are, for example, compounds of yttrium that are soluble in the solvent mixtures.
A significant advantage of the process according to the invention, and also of the zirconium-containing powder that is used in this process and of the coating solution that is produced therefrom, can be seen in the fact that the powder has a high solids content with respect to ZrO2 and is capable of being stored in contact with air for an unlimited period. The powder dissolves very easily in residue-free manner in numerous solvents and solvent mixtures. Many sol properties that are essential for the coating process can be adjusted selectively through the choice of the solvent or solvent mixture, without the synthesis of the powder having to be changed. In addition, the layer thicknesses that can be obtained with a single coating are very considerable.